If the user is dragging the scrollbar thumb and (perhaps accidentally) uses the mousewheel, the desired behavior is platform-specific:
* On Mac, native scrollbars seem to ignore the wheel events.
* On Windows, native scrollbars snap back to the drag origin until the user moves the mouse again.
Currently WebCore does neither of these (on Windows it applies the wheel scroll and continues the drag; not sure what Safari version I tested on Mac so I won't comment there).
Additional testing on Windows shows that the snap-back gets reverted on mouseup even if the mouse doesn't move any more. This means that the snap-back is basically completely nonfunctional and just looks broken. I don't care about matching this behavior, it looks more like a bug than a feature.
Conceivably, we should ignore wheel-while-dragging, but whatever.